1. Field of the Invention
The present invention relates to a hydraulically operated power steering system for use in an automotive vehicle and particularly to a hydraulically operated power steering system having a valve spool which changeovers the fluid communication between each fluid chamber of a reciprocal hydraulic operating cylinder device and a fluid pump or a reservoir in response to the rotation of an input shaft and which is disposed at the outside of a power piston of the reciprocal hydraulic operating cylinder device.
2. Description of the Related Art
It is known that there are two types of a hydraulically operated power steering systems. One type of hydraulically operated power steering system includes a valve spool which changeovers the fluid communication between each hydraulic pressure chamber of a reciprocal hydraulic operating cylinder device and a fluid pump or a reservoir in response to the rotation of an input shaft. The valve spool is disposed at the outside of a power piston of the reciprocal hydraulic operating cylinder device which is slidably fitted in a housing, and a valve lever is disposed in the power piston so as to be relatively rotatable with respect to the power piston and to move with the power piston in response to the rotation of the input shaft and engages with the valve spool. This type of power steering system has the advantages of another type in which the valve spool is disposed in the power piston so as to reduce the weight of the power piston and simplify the structure of the hydraulically operated power steering system. For example, U.S. Pat. Nos. 3,885,456 and 3,990,350 disclose this type of system.
In this type of system, since the valve lever is arranged for unitary movement with the power piston, if an engaging portion of the valve lever is constructed so as to not project outside of the power piston and a bore, fitted with the valve spool, is overlapped to an inner bore of the housing fitted the power piston, the axial length of the power piston is increased in order to ensure the length of the guide surface of the power piston.
As a result, since the axial length of the hydraulically operated power steering system is increased, it is necessary to project the engaging portion of the valve lever to the outside of the power piston and to separate the bore fitted with the valve spool from the inner bore fitted with the power piston in order to prevent an increase in the length of the system. Further, since the power piston is provided with a rack portion which transmits the movement of the input shaft to the output shaft and which is not a guide surface of the power piston, it is necessary to position the valve lever in the same position in the axial direction with respect to the position of the rack portion. Furthermore, in this case, it must be considered that the power piston is disposed in the inner bore of the housing after the valve lever is received in the power piston in order to provide for ease of installation.
Therefore, due to these restrictions, in the above prior system, the inner bore of the housing and the power piston are formed in a stepped-shape. That is, the inner bore has a large-diameter portion at its opening side so as to not be detrimental or otherwise hinder the ease of installation, and the rack portion and the valve lever are positioned in a small-diameter portion of the power piston. Further, the engaging portion of the valve lever is received in an outer diameter, i.e., the large-diameter portion of the power piston. According to this structure, the axial length of the system is reduced. However, since the rack portion is formed on the small-diameter portion of the power piston, the strength of the rack portion cannot be ensured due to shortening or decreasing the axial length of the power piston by simply positioning the valve lever and the rack portion at the same position in the axial direction when the magnitude of the power assist of the steering system is increased for use in large-size vehicles.